In recent years, as computers have developed and become popular, various kinds of information are put into digital data. As a device for storing such digital data, there is a storage device such as a magnetic tape and a magnetic disk. Because data to be stored has increased day by day and the amount thereof has become huge, a high-capacity storage system is required. Moreover, it is required to keep reliability while reducing the cost of a storage device. In addition, it is required that data can be retrieved later with ease. As a result, such a storage system is desired that is capable of automatically realizing increase of the storage capacity and the performance, that eliminates duplicated storage to reduce the cost of storage, and that has high redundancy.
Under such circumstances, in recent years, a content address storage system has been developed as shown in Patent Document 1. This content address storage system distributedly stores data into a plurality of storage devices, and specifies a storing position in which the data is stored based on a unique content address specified according to the content of the data. To be specific, the content address storage system divides given data into a plurality of fragments, adds a fragment to become redundant data thereto, and stores these fragments into a plurality of storage devices, respectively.
Later, by designating a content address, it is possible to retrieve data, namely, a fragment stored in a storing position specified by the content address, and restore the given data before being divided from the plurality of fragments.
Further, the content address is generated so as to be unique according to the content of data. Therefore, in the case of duplicated data, it is possible to refer to data in the same storing position to thereby acquire data with the same content. Accordingly, it is not necessary to separately store the duplicated data, and it is possible to eliminate duplicated recording and reduce the amount of data.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. JP-A 2005-235171
However, in the content address storage system that eliminates duplicated recording as described above, in a case that a given data block having been stored is lost, all the other data referring to the data block are affected. For example, assuming a data block with a reference count of 100 exists, this data block is the same data as the other one-hundred data blocks and hence the one-hundred data blocks do not exist and only the one data block is stored. Therefore, in a case that this data block with a reference count of 100 is lost, the other one-hundred pieces of logical data referring to the data block are lost.
Accordingly, in order to increase the fault tolerance of stored data, the content address storage system generally executes a process for maintaining the redundancy degree of each data block high. For example, in order to secure a certain redundancy degree that is previously set, the content address storage system stores data blocks provided with redundant data. However, such a redundancy degree is uniformly given to all of the data blocks. That is to say, for example, the redundancy degree of a data block with a reference count ‘1’ and the redundancy degree of a data block with a reference count ‘100’ are equally handled. In such a case, the data block with the reference count ‘100’ has a larger influence on the stored data, but the same redundancy degree and storing region are also given to the data block with the reference count ‘1.’